The invention relates to electronic devices, and, more particularly, to formation of metal (such as copper and copper alloy) structures useful in integrated circuits and printed wiring boards.
A steady demand for increasing performance in the electronics industry has driven integrated circuit technology to faster operation and more densely packed circuits. This translates to smaller devices and smaller propagation delays. In particular, the cross section of conductive interconnections among devices decreases with circuit downsizing, but an increase in their resistance and stray capacitance will degrade propagation delay times. This has led to the replacement of polysilicon interconnections with more conductive materials such as silicided polysilicon, aluminum, and tungsten. Indeed, aluminum alloy interconnections for top metal level is industry standard; but large current densities used in small cross sectional aluminum interconnections leads to electromigration, stress migration, and voiding where aluminum alloy lines become fractured or otherwise degraded during operation. Further, metal interconnections must provide low resistance to avoid propagation delay due to the increasingly dominant influence of RC delays in circuit performance at sub half micron line widths. Currently, aluminum in the form of roughly 2% copper (to enhance electromigration resistance) has a resistivity of about 3-4 .mu.ohm-cm. Tungsten has good electromigration resistance but has a resistivity of about 5.4 .mu.ohm-cm. In contrast, copper has good electromigration resistance and a resistivity of about 1.7 .mu.ohm-cm.
Despite the advantages of copper with respect to resistivity and electromigration, it has not been widely used in integrated circuits due to factors such as (i) copper forms a deep level acceptor in silicon which will reduce minority carrier lifetime, (ii) copper will react with silicon dioxide to form copper oxide and silicon at high temperatures, (iii) copper has a high diffusion coefficient in silicon dioxide, and (iv) the lack of a suitable copper dry etch process. However, advances in diffusion barriers such as TiN and low temperature deposition of copper has alleviated problems (i)-(iii), but finding a suitable etch remains a problem.
The development of low temperature deposition of copper by chemical vapor deposition (CVD) includes approaches to selective deposition of copper, and the deposition reactions run in reverse possibily provide etches. For example, Jain et al, Control of Selectivity During Chemical Vapor Deposition of Copper from Copper (I) Compounds via Silicon Dioxide Surface Modification, 61 Appl. Phys. Lett. 2662 (1992), discloses selective CVD of copper from a disproportionation reaction such as EQU 2 Cu(hfac)P(CH.sub.3).sub.3 (g).fwdarw.Cu(s)+Cu(hfac).sub.2 (g)+2 P(CH.sub.3).sub.3 (g)
where hfac is 1,1,1,5,5,5-hexafluoroacetylacetonate. The copper in the lefthand portion of the equation is cuprous (+1 valence) and the copper on the righthand side is copper metal and cupric (+2 valence). The trimethylphosphine is an adducted Lewis base, and both oxygens in the hfac bond to the copper. Recall that the coordination number of copper is most commonly 4 for cuprous and 4 or 6 for cupric. U.S. Pat. Nos. 5,098,516 and 5,221,366 disclose the foregoing and other deposition and etching of copper with diketone compounds.
In general, seven methods have been or are used to pattern thick copper films: (1) plasma etch processing of blanket copper films, in most instances using a chlorine-based etch chemistry, (2) ion milling or magnetron etching blanket copper films with an argon or chlorine reagent, (3) wet chemical etching blanket copper films with ferric chloride or nitric acid, (4) dry etching blanket copper films with activated gaseous reagents such as hfac or other organic radicals, (5) selective CVD of copper as described in the preceding paragraph, (6) selectively electroplating a copper film on predisposed conductor regions, and (7) metal lift-off. These approaches have problems for small linewidths as follows. Plasma etching has generally failed due to the lack of a copper halide with reasonable vapor pressure at 50.degree.-100.degree. C. Higher temperatures lead to particle contamination in cold wall reactors and the etch exhibits significant isotropy.
Ion milling or magnetron etching with argon or chlorine boils down to a mere sputter removal of the copper film, and this redeposits on the reacator walls, leading to particulate contamination and device damage.
Wet etching of copper films works well at large feature sizes, but wet etches severely undercut the copper film with respect to a masking layer. Small feature sizes cannot tolerate the isotropy or the contamination introduced by wet chemicals.
Dry etching with gaseous reagents falls into different classes. A first class introduces plasma activated organic radicals, such as methyl redicals, to react with copper to form a solid or a gas product that is removed either by a wet solution or by evolution from the surface as a vapor. However, the organic precursors can polymerize in an energetic etch chamber and contaminate the reactor and the copper surface, terminating the reaction with copper. A second class introduces ligands, such as the hfac and trimethylphosphine previously described, to react with copper forming volatiles which then evaporate from the surface. That is, the preceding deposition reaction may be run in reverse by passing Cu(hfac).sub.2 and P(CH.sub.3).sub.3 over the copper to be etched. A third class intially chlorinates the copper to be etch to form CuCl.sub.x and then passes P(C.sub.2 H.sub.5).sub.3 over the CuCl.sub.x to form CuCl(P(C.sub.2 H.sub.5).sub.3).sub.2 which vaporizes in a low pressure chamber. See Farkas et al, Low-Temperature Copper Etching via Reactions with Cl.sub.2 and PEt.sub.3 under Ultrahigh Vacuum Conditions, 73 J. Appl. Phys. 1455 (1993). Such processes tend to be isotropic and may require high temperatures to increase volatility. The vapor pressure of these copper ligand products are low at room temperature. The ligands are fairly complex molecules that are easily fragmented and rendered useless as etch reagents in an energetic (e.g., plasma, afterglow, . . . ) dry etch chamber.
Selective copper CVD and selective copper electroplating both require a seed layer, and this seed layer itself may require etching to create the pattern. Again practical etching of copper is lacking. Lastly, copper lift-off has limited use at submicron feature sizes with dense patterns.
Thus the formation of patterned copper films has problems such as small feature etching including isotropy.
Plasma etching of silicon can be enhanced through time modulation of the plasma or of the etching gasses. For example, Boswell et al, Etching in a Pulsed Plasma, 62 J. Appl. Phys. 3123 (1987), describes silicon etching with SF.sub.6 and pulsed RF power having a 20% duty cycle and pulse durations down to 1 ms. The etch rate rise time when the plasma is excited was about 2 ms, and the etch rate decay time when the plasma was extinguished was about 50 ms. Similarly, McNevin, Radio Frequency Plasma Etching of Si/SiO.sub.2 by Cl.sub.2 /O.sub.2 : Improvements Resulting from the Time Modulation of the Processing Gases, 9 J. Vac. Sci. Tech. A 816 (1991), describes pulsing the oxygen in a chlorine/oxygen etch of silicon with oxygen pulses as short as 60 ms in a parallel plate etcher with a gas residence time of only 25 ms.